The actress attacked the ruling banning the LighterLife ad, saying: "Obesity is increasing, people are dying from cancer, heart disease and diabetes and yet a brilliant solution is being kept from us."
The social media page featured before and after images of the actress and said: "Denise lost two stone in just two months."
Responding to a complaint challenging whether the ad complied with rules on weight-loss advertising, LighterLife described Welch as overweight with a "very high" waist circumference when she began her programme, adding that she was at high risk of health problems.
The company said Welch lost more than 2lb (around 900g) a week while on the programme.
But it said that Welch's rate of weight loss was in line with expected results for those on a very low calorie diet (VLCD) programme and, while the rate was greater than 2lb a week, this did not mean that it was incompatible with good medical and nutritional practice.
The company said Nice guidelines stated that a diet of less than 600kcal a day should be used only under clinical supervision, but the minimum daily intake for those on a VLCD LighterLife programme was greater than 600 kcal and those who required medical supervision received it, in line with company policy.
The Advertising Standards Authority (ASA) noted that the Committee of Advertising Practice code stated that claims for an individual to have lost an exact amount of weight must be compatible with good medical and nutritional practice.
The ASA said: "We understood that Denise Welch had been classed as overweight when her weight loss programme began and also that her waist circumference was very high, which put her at high risk of health problems.
"However, the code specified that for those who were normally overweight, a rate of weight loss greater than 2lb a week was unlikely to be compatible with good medical and nutritional practice.
"While we understood that LighterLife had given an account of the actual rate of weight loss experienced by Ms Welch, we noted the rate advertised was greater than 2lb per week.
"We had not seen evidence to demonstrate that the rate of loss experienced by Ms Welch was compatible with code requirements."
The ASA ruled that the ad must not appear again in its current form, adding: "We told LighterLife to ensure that rates of weight loss were compatible with good medical and nutritional practice and that treatments for obesity were not advertised to the public without suitably qualified supervision."
Responding to the ruling, Welch said: "The Advertising Standards people need to get out more.
"Weight loss companies are trying to help people, and yet they aren't even allowed to tell us what they can do for us.
"And LighterLife has been prevented from using my weight loss before and after photos as the ASA says I lost weight too quickly. How can that be right, when the Foodpacks comply with all the nutritional rules, including Trading Standards, and I did exactly what I was supposed to do with them.
"I felt better than I had for ages, and those are genuine before and after pictures.
"Meanwhile obesity is increasing, people are dying from cancer, heart disease and diabetes and yet a brilliant solution is being kept from us."
LighterLife medical director Dr Matt Capehorn said: "This shows just why Britain is in its current mess, with one of the biggest obesity problems on the planet.
"There is something wrong when weight loss companies are trying to help dieters, and bureaucracy gets in the way."